Radiation sensor

ABSTRACT

A radiation sensor includes a radiation source, at least two scintillators having distinguishable scintillation-light-emission-decay characteristics, and shielding configured to provide radiation-paths of different shielding between the source and the at least two scintillators. The shielding is configured such that the scintillators are primarily sensitive to radiation passing through different paths. The signals produced from scintillation events can be sorted by the scintillator that is the source of the event. A source-scintillator-based comparison of the signals may be used to provide information about the material in one or more of the radiation paths. The sensor may be disposed within a borehole to measure the density of the fluid in the borehole.

BACKGROUND

This invention pertains generally to technology for sensing andmeasuring electromagnetic radiation (photons) using scintillatordevices. More particularly, the invention pertains to a configuration ofscintillation material, shielding, and radiation source that may be usedto measure properties of material in or around a wellbore (borehole).For example, the invention may be used to efficiently measure thedensity of fluid in the borehole when deployed in a wireline or otherlogging tool.

Prior-art systems for the measurement of fluid density in producing oiland gas wells are based on a sensor with a radio-isotope source such as²⁴¹Am. Radiation from the source traverses the borehole fluid in an opensample cell and the counts received by a detector on the opposing end ofthe sample cell are dependent on the fluid density. In these prior-artsystems the use of radio-isotopes cause concerns for health and safetyof personnel and security of radio-isotopes.

Miniature x-ray tubes are viable alternate radiation sources if theoutput of the tube can be accurately measured. In laboratory andindustrial applications, a separate output-monitor detector is added tomonitor tube output. Use of a separate output-monitor detector for insitu measurements in a borehole is problematic due to space constraintsthat constrain sensors to a small diameter and long aspect.

SUMMARY

This invention uses the geometry of the sample cell to focus certainradiation paths on certain parts of a multi-faceted detector. Each facetof the detector is a certain scintillation crystal with a specificemission time, thus the light received by a photomultiplier tube can besorted by species and related to a specific path of the radiation.Depending on sample cell design and detector faceting, the invention canbe configured to measure: tube output, transmission through the wellborefluid, and back-scattering of radiation by the wellbore fluidsurrounding the sensor.

In one aspect of the invention, a photon sensor includes at least twoscintillators (e.g., YSO, CaF₂), each having scintillation-light-timecharacteristics distinguishable from scintillation-light-timecharacteristics of the other scintillators. These scintillators are eachcoupled to a common photodetector (e.g., photomultiplier tube). Thesignals generated by the photodetector (and support electronics) havetiming characteristics reflective of the scintillation-light-timecharacteristics and are thereby distinguishable. The photon sensorfurther includes shielding configured to enhance the sensitivity of thescintillators to photons travelling along certain paths as compared tophotons traveling other paths. Different scintillators are sensitive todifferent paths. The photon sensor further includes a pulse-shapediscriminator capable of distinguishing signals based on thescintillator in which the signal originates.

In another aspect of the invention, a fluid-density logging toolincludes at least two scintillators (e.g., YSO, CaF₂), each havingscintillation-light-time characteristics distinguishable fromscintillation-light-time characteristics of the other scintillators.These scintillators are each coupled to a common photodetector (e.g.,photomultiplier tube). The signals generated by the photodetector (andsupport electronics) have timing characteristics reflective of thescintillation-light-time characteristics and are therebydistinguishable. The logging tool further includes a means for enhancingthe sensitivity of the different scintillators to different radiationpaths. At least one scintillator is sensitive to a path through materialof a known density and at least one scintillator is sensitive to a paththrough material of an unknown density. The logging tool furtherincludes a means for distinguishing signals based on the scintillator inwhich the signal originates.

In another aspect of the invention, a method for determining the densityof a material includes detecting photons with at least one of multiplescintillators, each scintillator having a characteristicscintillation-light-decay time that is distinguishable from thecharacteristic scintillation-light-decay time of the otherscintillators. The method further includes determining whichscintillator detected the photon based on the characteristicscintillation-light-decay time. The method further includes combiningthe number of events detected based on the detecting scintillator (e.g.,dividing the number of events in one scintillator by the number ofevents in a second scintillator). The method further includes applying acalibration to the combination to provide a measure of the density ofthe material through which the photons detected by at least one of thescintillators passed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will be become better understood with reference to thefollowing description, appended claims, and accompanying drawings where:

FIG. 1 illustrates an exemplary x-ray sensor integrated into afluid-density tool disposed in a borehole.

FIGS. 2a-2c are sectional views of an exemplary two-scintillator x-raysensor integrated into a fluid-density tool disposed in a borehole.

FIG. 3 is an exploded isometric view of an exemplary scintillatorassembly of an exemplary x-ray sensor.

FIG. 4 is a partially exploded isometric view of an exemplary detectorassembly of an exemplary x-ray sensor.

FIG. 5 illustrates a portion of a fluid-density tool including anexemplary x-ray sensor.

FIG. 6 illustrates an exemplary operational flow of a fluid-density toolincluding an exemplary two-scintillator x-ray sensor.

FIG. 7 illustrates an exemplary operational flow of a fluid-density toolincluding an exemplary two-scintillator x-ray sensor.

FIGS. 8a-8c are sectional views of an exemplary two-scintillator x-raysensor integrated into a fluid-density tool disposed in a borehole.

FIG. 8d illustrates a portion of a fluid-density tool including anexemplary x-ray sensor.

FIGS. 9a-9c are sectional views of an exemplary three-scintillator x-raysensor integrated into a fluid-density tool disposed in a borehole.

FIG. 9d illustrates a portion of a fluid-density tool including anexemplary x-ray sensor.

FIGS. 10a-10c are sectional views of an exemplary three-scintillatorx-ray sensor integrated into a fluid-density tool disposed in aborehole.

FIG. 10d illustrates a portion of a fluid-density tool including anexemplary x-ray sensor.

FIG. 11 illustrates an exemplary operational flow of a fluid-densitytool including an exemplary three-scintillator x-ray sensor.

FIGS. 12a and 12b illustrate an exemplary single-channel analyzer thatmay be used in the invention.

FIGS. 13a and 13b illustrate an exemplary multi-channel analyzer thatmay be used in the invention.

FIG. 14 illustrates and exemplary detector-signal-processing circuitthat may be used in the invention.

FIG. 15 illustrates and exemplary flow for detector-signal processing ina processor circuit.

FIGS. 16a-16d illustrate exemplary timing diagrams for an exemplarymulti-channel analyzer that may be used in the invention.

FIG. 17a illustrates an exemplary timing diagram for an exemplarysingle-channel analyzer that may be used in the invention.

FIG. 17b illustrates an exemplary processing of a digitized detectorsignal to determine timing characteristics of the detector signal (andthus determine the scintillator source of the signal).

FIGS. 18a-18b illustrate exemplary detector responses for an exemplaryfluid-density instrument.

DETAILED DESCRIPTION

In the summary above, and in the description below, reference is made toparticular features of the invention in the context of exemplaryembodiments of the invention. The features are described in the contextof the exemplary embodiments to facilitate understanding. But theinvention is not limited to the exemplary embodiments. And the featuresare not limited to the embodiments by which they are described. Theinvention provides a number of inventive features which can be combinedin many ways, and the invention can be embodied in a wide variety ofcontexts. Unless expressly set forth as an essential feature of theinvention, a feature of a particular embodiment should not be read intothe claims unless expressly recited in a claim.

Except as explicitly defined otherwise, the words and phrases usedherein, including terms used in the claims, carry the same meaning theycarry to one of ordinary skill in the art as ordinarily used in the art.

Because one of ordinary skill in the art may best understand thestructure of the invention by the function of various structuralfeatures of the invention, certain structural features may be explainedor claimed with reference to the function of a feature. Unless used inthe context of describing or claiming a particular inventive function(e.g., a process), reference to the function of a structural featurerefers to the capability of the structural feature, not to an instanceof use of the invention.

Except for claims that include language introducing a function with“means for” or “step for,” the claims are not recited in so-calledmeans-plus-function or step-plus-function format governed by 35 U.S.C. §112(f). Claims that include the “means for [function]” language but alsorecite the structure for performing the function are notmeans-plus-function claims governed by § 112(f). Claims that include the“step for [function]” language but also recite an act for performing thefunction are not step-plus-function claims governed by § 112(f).

Except as otherwise stated herein or as is otherwise clear from context,the inventive methods comprising or consisting of more than one step maybe carried out without concern for the order of the steps.

The terms “comprising,” “comprises,” “including,” “includes,” “having,”“haves,” and their grammatical equivalents are used herein to mean thatother components or steps are optionally present. For example, anarticle comprising A, B, and C includes an article having only A, B, andC as well as articles having A, B, C, and other components. And a methodcomprising the steps A, B, and C includes methods having only the stepsA, B, and C as well as methods having the steps A, B, C, and othersteps.

Terms of degree, such as “substantially,” “about,” and “roughly” areused herein to denote features that satisfy their technological purposeequivalently, though perhaps not identically, to a feature that is“exact.” For example, a component A is “substantially” perpendicular toa second component B if A and B are at an angle such as to equivalentlysatisfy the technological purpose of A being perpendicular to B.

Except as otherwise stated herein, or as is otherwise clear fromcontext, the term “or” is used herein in its inclusive sense. Forexample, “A or B” means “A or B, or both A and B.”

Electromagnetic radiation (photons) such as x-rays and gamma-rays, as iswell known in the art, interacts with material through a number ofmechanisms, such as Rayleigh scattering, Compton scattering, thephotoelectric effect, pair production, and photonuclear absorption. Thecombined effect of these interactions on a beam of photons passingthrough a layer of material can be represented by the followingequation:I=I ₀ e ^(−μρx)Here, I is the intensity of the photon beam exiting the material, I₀ isthe intensity of the photon beam incident on the material, μ is themass-attenuation coefficient of the material, ρ is the density of thematerial, and x is the thickness of the layer of material. Themass-attenuation coefficient is a function of the material and of theenergy of the incident photon. For materials consisting of homogenousmixtures or compounds of atomic constituents, the mass-attenuationcoefficient of the mixture or compound is the weighted sum of themass-attenuation coefficients of the constituents:=Here, w_(i) is the fraction by weight of the i^(th) constituent and isthe mass-attenuation coefficient of the i^(th) constituent. Thus, ameasure of the attenuation of a photon beam travelling through amaterial provides information about the density and makeup of thematerial:

${- {\ln\left( \frac{I}{I_{0}} \right)}} = {\left( {\mu\; x} \right)\rho}$

Electromagnetic radiation such as x-rays or gamma-rays, as is well knownin the art, may be provided by a chemical source or by a generator.Chemical sources provide the photons through radioactive decay of anisotope. The intensity and energy distribution of the photons is afunction of the isotope. Generators, typically comprising a tube andcontrol circuitry, provide photons by accelerating a charged particle(e.g., an electron) through an electric potential (a voltage difference)to collide with a target in the tube. The kinetic energy of the chargedparticle is converted in part to photonic energy. The intensity of thephoton beam produced at the target is a function of the number ofaccelerated charged particles (the current) and the maximum energy ofthe photons in the beam is a function of the potential through which theparticles are accelerated (the voltage). Thus, by controlling thecurrent and voltage of the generator, an operator may control theintensity and energy distribution of the photon beam.

Scintillators, as is well known in the art, are materials that producelight when excited by ionizing radiation. For example, ionizingelectromagnetic radiation such as x-ray or gamma-ray photons interactwith the material to place the material into an excited energy state.Deexcitation of the material produces light (e.g., photons in thevisible or near-visible range). The characteristics of the excitationand deexcitation process, and thus the characteristics of the lightemission, depend in part on the scintillation material. For example, theamount of scintillation light produced by the scintillator per unit ofenergy deposited by the ionizing radiation varies from scintillatormaterial to scintillator material. Similarly, the time variance of thescintillation light produced by the scintillator varies fromscintillator material to scintillator material. For many materials, thistime variance may be approximated as short period of excitation followedby a longer period in which the intensity (amount) of scintillationlight produced decays exponentially:

${I_{S}(t)} = {I_{{S\_}0}\left( {e^{- \frac{t}{\tau_{D}}} - e^{\frac{t}{\tau_{B}}}} \right)}$Here, Is(t) indicates the intensity of scintillation output as afunction of time (t), I_(S) _(_) ₀ indicates the maximum intensity,τ_(D) indicates the characteristic light-decay time, and τ_(B) indicatesthe characteristic excitation time. Thus, a scintillator may bedistinguished from another scintillator based on the shape or width ofthe scintillation light pulse. Common inorganic scintillators include,for example, NaI(Tl), CsI(Tl), BGO, CaF₂(Eu), BaF₂, YAP(Ce), YSO(Ce),LYSO(Ce), LSO(Ce), and LaCl₃(Ce). There are also a variety of organicscintillators.

Electronic photodetectors, or light sensors, as is well-known in theart, are devices that convert light (photons in the visible ornear-visible range) into electronic signals (e.g., voltage or currentpulses). Photomultiplier tubes and photodiodes are examples ofelectronic photodetectors.

Pulse-shape discriminators (PSDs), including pulse-width discriminators(PWDs), refer to a well-known class of signal-processing structures.These structures may be implemented in hardware or software or somecombination thereof. These structures use timing characteristics of asignal, such as rise time, decay time, and width, to categorize asignal. For example, a pulse-shape discriminator may categorize allsignals according to the duration of the signal's decay tail. And apulse-width discriminator may categorize signals according to the fullwidth at half maximum of the signal. Signals with a common timingcharacteristic falling within a particular range of durations areconsidered of the same category.

Single-channel analyzers (SCAs) and multi-channel analyzers (MCAs) referto a class of well-known signal-processing structures. These structuresmay be implemented in hardware or software or some combination thereof.These structures use magnitude characteristics of a signal to categorizea signal. A single-channel analyzer counts all signals above a lowerthreshold and below and upper threshold as a single category. Amulti-channel analyzer will divide the range of magnitudes within theupper and lower thresholds into multiple categories (ranges ofmagnitudes) and separately count the signals according to the magnituderange they fall within.

An exemplary fluid-density instrument incorporating an exemplary x-raysensor according to an embodiment of the invention is shown in FIG. 1.The instrument is depicted attached to a wireline 19 and disposed in aborehole defined by a casing 17 and a borehole fluid 15. A scintillatorassembly 14 is optically coupled to a photomultiplier tube 16 a which iselectrically or magnetically coupled to pulse-processing circuitry 16 b.The scintillator assembly 14 is positioned within a section of a housing11 and spaced apart from an x-ray source 10 b. The x-ray source 10 b,here an x-ray generator tube, is controlled through control circuitry 10a. The x-ray source 10 b and control circuitry 10 a are positionedwithin a section of the housing 11. A measurement cage comprising a tube12 and support member 13 is positioned between the scintillator assembly14 and the x-ray source 10 b. The cage includes a volume between thesupport member 13 and the outside surface of the tube 12 that is open toborehole fluid 15. The inside surface of the tube 12 defines a volumethat excludes the borehole fluid 15 (but may include material of a knowndensity such as boron carbide or nitrogen). X-rays originating at thex-ray source 10 b travel through the various materials defining theborehole and instrument environment. Some x-rays travel through thefluid-excluding volume of the tube 12 to the scintillator assembly 14where they are detected. Other x-rays travel through the fluid-filledvolume of the cage to the scintillator assembly 14 where they aredetected. Measurement of characteristics of the detected x-rays (e.g.,number of x-rays detected per unit time and the energy distribution ofthe detected x-rays) can provide information regarding characteristicsof the borehole environment, such as the density of the borehole fluid.

The exemplary scintillator assembly 14 can be understood with referenceto FIG. 2a (a view of section A-A′ of FIG. 1) and FIG. 3 (an explodedview of a variant of the assembly 14). The scintillator assembly 14includes a first scintillator element 14 a and a second scintillatorelement 14 b. The first scintillator element 14 a is configured in aring shape (roughly a disk with a hole through the center) such that thesecond scintillator element 14 b fits within the hole defined by thefirst scintillator element 14 b. Scintillator assembly 14 is a coaxialassembly of the first and second scintillator elements 14 a, 14 b. Anx-ray shield 14 c may optionally be coaxially disposed between the firstscintillator element 14 a and the second scintillator element 14 b.

The materials of the first and second scintillator elements 14 a, 14 bdiffer with respect to the time characteristics of the scintillationlight output. The idealized plots of scintillation light intensityversus time (light pulses) 31, 32 depicted in FIG. 3 show scintillationmaterials that differ primarily with respect to light decay time. The“fast” scintillation material of the second scintillator element 14 bproduces all its scintillation light corresponding to a scintillationevent (an interaction of an x-ray with the scintillation material) in ashorter period of time than does the scintillation material of the firstscintillator element 14 a. This scintillation-time-characteristicdistinction between the first scintillator element 14 a and the secondscintillator element 14 b allows for distinguishing events in the firstscintillator element 14 a from events in the second scintillator element14 b based on light-pulse shape. Thus, as shown in FIG. 4 (a partiallyexploded view of the coupled scintillator assembly 14 andphotomultiplier tube 16 a), a common photomultiplier tube 16 a can beused to convert scintillation light from the assembly 14 to electronicsignals for processing by pulse-processing circuitry 16 b whilepreserving the ability to identify the source of the scintillation eventas either in the first scintillator element 14 a or the secondscintillator element 14 b.

The materials of the tube 12, support members 13, and housing 11 arechosen and structured to shield portions of the scintillator assembly 14from x-rays that travel through one path less than from x-rays thattravel through different paths. For example, as depicted in FIGS. 2a,2b, and 2c (sectional views of section A-A′, B-B′, C-C′ of FIG. 1,respectively) and FIG. 5 (a view of a portion of FIG. 1), the housing 11and tube 12 may be structured such that: (1) the first scintillatorelement 14 a is more sensitive to x-rays traveling through boreholefluid 15 than to x-rays traveling through the fluid-excluding volume 12b of the tube 12 and (2) the second scintillator element 14 b is moresensitive to x-rays traveling through the fluid-excluding volume 12 b ofthe tube 12 than to x-rays traveling through the borehole fluid 15. Thesupport member 13 comprises three support-member elements 13 a, 13 b, 13c that provide the necessary structural support while allowing theborehole fluid 15 to occupy the space between the casing 17 and theouter surface of the tube 12. The tube 12 comprises the annular material12 a and the fluid-excluding volume 12 b. The portion 11 a of thehousing 11 positioned between the source 10 b and scintillator assembly14 includes several separated longitudinally-thinner sections (along thewireline tool's longitudinal axis) that define further volumes 15 a, 15b, 15 c open to borehole fluid 15. The portion 11 b of the housing 11positioned coaxially around the scintillator assembly 14 is of uniformradial thickness. The combination of the wall 12 a of the tube 12, theportion 11 a of the housing 11, and the portion 11 b of the housing 11provides the first scintillator element 14 a with less shielding fromx-rays that travel from the x-ray generating target 10 c of the source10 b through the volume of borehole fluid 15 radially within the supportmember 13 volume than from x-rays that travel other paths. The samecombination provides the second scintillator element 14 b with lessshielding from x-rays that travel from the x-ray generating target 10 cof the source 10 b through the fluid-excluding volume 12 b of the tube12 than from x-rays that travel other paths. Thus, (1) events in thefirst scintillator element 14 a primarily indicate interactions ofx-rays with the borehole fluid, and (2) events in the secondscintillator element 14 b primarily indicate the output of the source.The rates of events (intensity) in the two scintillator elements 14 a,14 b may be combined to provide an indication of x-ray attenuation, andthus borehole-fluid density.

The operation of the fluid-density instrument of FIG. 1 may beunderstood with reference to the exemplary flow depicted in FIG. 6. Whenthe instrument is in the desired location in the borehole, themeasurement is started 60. X-rays that travel from the source 10 b tothe scintillator assembly 14 may interact with the scintillator assembly14 to create a scintillation event in the first scintillator element 14a or the second scintillator element 14 b. The light pulse of thescintillation event is collected 62 a when the photomultiplier tube 16 aconverts the scintillation light pulse to an electronic signal havingtime characteristics reflective of those of the light pulse. Theprocessing circuitry 16 b, via a pulse-width discriminator, categorizes63 the signal according to its width (and thus categorizes the event asin the first scintillator element 14 a or the second scintillatorelement 14 b). The processing circuitry 16 b, via a single-channelanalyzer (or, optionally, a multi-channel analyzer), then increments theappropriate counter 64 a, 64 b. A single-channel analyzer for the firstscintillator element 14 a (the “slow” scintillation material) will count64 b all events in the first scintillator element 14 a that also yield apulse height within a range defined by a lower-level discriminationthreshold and an upper-level discrimination threshold. A single-channelanalyzer for the second scintillator element 14 b (the “fast”scintillation material) will count 64 a all events in the secondscintillator element 14 b that also yield a pulse height within a rangedefined by a lower-level discrimination threshold and an upper-leveldiscrimination threshold. Multi-channel analyzers may be used to countevents as a function of pulse height. The collection of pulses andcounting of events repeats 62 b for some interval of time or depth inthe borehole.

When counting for a given interval is complete, the fast counts and slowcounts are combined 65. For example, the total count of events in thefirst scintillator element 14 a may be divided by the total count ofevents in the second scintillator element 14 b to generate a ratio. If amulti-channel analyzer is used, this ratio may be generated for variouspulse-height ranges. Other combinations are possible. For example, thedifference between the counts may be used. A calibration based on theborehole configuration is applied 66 and an answer is provided 67 forthe interval. For example, the ratio of first-scintillator-elementcounts to second-scintillator-element counts may be linearly related tothe density of the borehole fluid 15, with the parameters of the linearrelationship (slope and intercept) based on the inner diameter of thecasing 17. Other relationships between ratio and fluid density may beappropriate (e.g., a quadratic relationship, or a relationship with afirst linearity over a first range of densities and a second linearityof a second range of densities). Calibrations are typically determinedthrough a combination of computer modeling (e.g., Monte Carlo simulationof instrument response) and physical modeling (e.g., measurement ofvarious known fluid densities in various known borehole environments).Once the appropriate calibration is applied to the ratio, the fluiddensity may be provided by, for example, recording or displaying thedensity for the interval.

Once the desired measurement is complete, the process stops 69.

The flow of FIG. 7 is similar to the flow of FIG. 6. The differencebetween the flows is that the FIG. 7 flow allows for adjustment of thex-ray source 10 b. That is, the collecting step 72 a, categorizing step73, counting steps 74 a, 74 b, interval-check step 72 b, combining step75, calibration step 76, and answer step 77 are similar to,respectively, the collecting step 62 a, categorizing step 63, countingsteps 64 a, 64 b, interval-check step 62 b, combining step 65,calibration step 66, and answer step 67 described with reference to FIG.6. The difference being that the FIG. 7 flow may account for and utilizecontrol over the intensity or energy of the x-rays generated by thex-ray source 10 b while the FIG. 6 flow does not. Thus, the flow of FIG.6 is appropriate for a chemical source of x-rays and a generator-sourceof x-rays that is not controlled. The output of the x-ray source 10 b isset to a specific intensity or energy by providing the specification ofcurrent and voltage to the x-ray source control circuitry 10 a. Thus,the answer provided 77 in the FIG. 7 flow may include specification ofthe intensity or energy of the x-rays generated by the source 10 b. Forexample, based on the count of events in the second scintillator element14 b (the “fast” count), the intensity of x-rays produced by the source10 b may be adjusted up (e.g., to increase count rate and statisticalaccuracy) or down (e.g., to decrease pulse pile up due to a count ratetoo high for the scintillator elements or processing circuitry).

A variant of the measurement cage and shielding is shown in FIGS. 8a-8d. In this variant, the portion 11 a′ of the housing 11 positionedbetween the source 10 b and scintillator assembly 14 includes adisk-shaped longitudinally-thinner section (along the wireline tool'slongitudinal axis) that defines a volume 11 c′ of reduced shielding fromx-rays travelling through the borehole fluid 15 as compared to thevariant shown in FIGS. 2a-2c . And the portion 11 b′ of the housing 11positioned coaxially around the scintillator assembly 14 has separatesections of radially-thinner material to reduce the shielding fromx-rays travelling through the borehole as compared to the variant shownin FIGS. 2a-2c . The shielding effects are similar to that describedwith reference to FIGS. 2a-2c : (1) events in the first scintillatorelement 14 a primarily indicate interactions of x-rays with the boreholefluid, and (2) events in the second scintillator element 14 b primarilyindicate the output of the source. The rates of events (intensity) inthe two scintillator elements 14 a, 14 b may be combined to provide anindication of x-ray attenuation, and thus borehole-fluid density.

A variant of the measurement cage, shielding, and scintillator assemblyis shown in FIGS. 9a-9d . In this variant, the scintillator assembly 14includes three concentric scintillator elements 14 a′, 14 b′, 14 d′,each with a distinctive scintillation-light-decay time (and thus eachwith a distinctive width of scintillation light pulse). The threeelements 14 a′, 14 b′, 14 d′ may be separated by shields similar to theshield 14 c depicted in FIG. 3. The portion 11 a″ of the housing 11positioned between the source 10 b and scintillator assembly 14 includesseveral separated longitudinally-thinner sections (along the wirelinetool's longitudinal axis) that define further volumes 15 a″, 15 b″, 15c″ open to borehole fluid 15. These volumes 15 a″, 15 b″, 15 c″ areradially positioned so that the third scintillator element 14 d′ is lessshielded from x-rays traveling through the borehole fluid in themeasurement cage than is the first scintillator element 14 a′ or thesecond scintillator element 14 b′. And the portion 11 b″ of the housing11 positioned coaxially around the scintillator assembly 14 has separatesections of radially-thinner material to reduce the shielding fromx-rays travelling through the borehole as compared to the variant shownin FIGS. 2a-2c . Thus, (1) events in the first scintillator element 14a′ primarily indicate interactions of x-rays with the borehole fluid inthe annulus between the fluid-density instrument and the casing 17, (2)events in the second scintillator element 14 b′ primarily indicate theoutput of the source, and (3) events in the third scintillator element14 d′ primarily indicate interactions of x-rays with the borehole fluidwithin the measurement cage.

A variant of the measurement cage, shielding, and scintillator assemblyis shown in FIGS. 10a-10d . In this variant, the scintillator assembly14 includes three scintillator elements 14 a″, 14 b″, 14 d″, each with adistinctive scintillation-light-decay time (and thus each with adistinctive width of scintillation light pulse). The first scintillator14 a″ and the third scintillator element 14 d″ are two halves of a ringpositioned around the second scintillator element 14 b″. The threeelements 14 a″, 14 b″, 14 d″ may be separated by shields. The portion 11a′″ of the housing 11 positioned between the source 10 b andscintillator assembly 14 is of a substantially uniform thickness. Andthe portion 11 b′″ of the housing 11 positioned coaxially around thescintillator assembly 14 has separate sections of radially-thinnermaterial to reduce the shielding from x-rays travelling through theborehole as compared to the variant shown in FIGS. 2a-2c . These thinnersections are positioned to align with the azimuthal position (relativeto the instrument's longitudinal axis) of the first scintillator element14 a″ and the third scintillator element 14 d″. Thus, (1) events in thefirst scintillator element 14 a″ primarily indicate interactions ofx-rays with the borehole fluid in the annulus between the fluid-densityinstrument and the casing 17 and on one side of the instrument, (2)events in the second scintillator element 14 b″ primarily indicate theoutput of the source, and (3) events in the third scintillator element14 d″ primarily indicate interactions of x-rays with the borehole fluidin the annulus between the fluid-density instrument and the casing 17and on the other side of the instrument. The rates of events (intensity)in the first and second scintillator elements 14 a″, 14 b″ may becombined to provide an indication of x-ray attenuation, and thusborehole-fluid density, for paths on one side of the instrument. Therates of events (intensity) in the third and second scintillatorelements 14 d″, 14 b″ may be combined to provide an indication of x-rayattenuation, and thus borehole-fluid density, for paths on the otherside of the instrument.

This may be useful, for example, for detecting multiphase borehole fluid(e.g., water and natural gas) when the phases separate. For instance,the instrument may be oriented in a deviated borehole such that thefirst scintillator element 14 a″ is facing down (i.e., oriented (atleast partially) along the gravitational force) and the thirdscintillator element 14 d″ is facing up (i.e., oriented (at leastpartially) against to the gravitational force). For borehole fluidcomprising natural gas and water, the natural gas may separate and riseto the upper side of the borehole and the water will fall to the lowerside of the borehole. Thus, the instrument may provide separate measuresof natural gas density and water density.

The operation of a three-scintillator-element sensor in a fluid-densityinstrument may be understood with reference to the exemplary flowdepicted in FIG. 11. This flow is similar to the flow depicted in FIG.7. The flow of FIG. 11 differs in that it discriminates among threedifferent pulse shapes (fast, medium, slow) 73′ and separately countsscintillation events for each pulse shape (and thus each of the threescintillator elements) 74 a′, 74 b′, 74 c′. These counts may be combined75′ in various ways (e.g., determining a ratio of slow to fast and aratio of medium to fast) and the various combinations may, throughapplication of calibrations 76′, be used to generate information aboutthe borehole environment (e.g., borehole fluid densities for each phasein a multiphase fluid).

An exemplary single-channel analyzer is depicted in the block diagram ofFIG. 12a . The single-channel analyzer includes two comparators 121,122, an AND gate 123, and a counter 124. The first comparator 121compares the signal from the detector (“signal,” in the figure) to anupper-level reference voltage (V_(up), the upper threshold). If theamplitude of the signal exceeds V_(up), then the first comparator 121output (V_(out) _(_) ₁) is a digital “high” or “1” and if the amplitudeof the signal is less than V_(up), then V_(out) _(_) ₁ is a digital“low” or “0.” The second comparator 122 compares the signal to alower-level reference voltage (V_(low), the lower threshold). If theamplitude of the signal exceeds V_(low), then the second comparator 122output (V_(out) _(_) ₂) is a digital “high” or “1” and if the amplitudeof the signal is less than V_(low), then V_(out) _(_) ₂ is a digital“low” or “0.” The AND gate 123 combines an inverted V_(out) _(_) ₁ withV_(out) _(_) ₂. If V_(out) _(_) ₂ is low (the signal amplitude is belowV_(up)) and V_(out) _(_) ₂ is high (the signal amplitude is aboveV_(low)), then the AND gate 123 output (V_(SCA)) is high, otherwise,V_(SCA) is low. The counter 124 counts the number of V_(SCA) pulses in agiven time interval, keeps those events that have only one count in theinterval, and thus counts the number of signal pulses having a maximumamplitude that falls within the voltage “channel” defined by V_(up) andV_(low). The time interval is a function of the width of the detectorsignal.

Optionally, the upper-level comparator may be omitted and the analyzermay process all events greater than the lower-level threshold. Alsooptionally, counts may be accepted or rejected based on a measurement ofthe signal width (e.g., to be accepted, the count event must correspondto a signal having a width within a predetermined range).

Multiple single-channel analyzers (or multiple counters in a singlesingle-channel analyzer), each with a distinct counting time interval,may be used to discriminate detector signals based on the width of thedetector signal. Thus, scintillation events may be distinguished basedon the scintillator that is the source of the event.

This single-channel analyzer may be equivalently implemented in softwareoperating on digitized detector signals. As is well-known in the art,the detector signal may be digitized by an analog-to-digital converter.The digitization may include periodic sampling (e.g., every 10 ns) tocapture the signal pulse amplitude at various points in time and thesoftware may be configured to determine the maximum amplitude (e.g., bycomparing sample amplitudes with other sample amplitudes to determinethe highest value sample amplitude). Alternately, the digitizationcaptures only a single sample indicative of the pulse signal's maximumamplitude. This maximum amplitude may then be compared to thresholdvalues encoded in the software (e.g., as a software variable). If themaximum amplitude is greater than the lower-level threshold and lessthan the higher-level threshold, a software counter (e.g., a softwarevariable) is incremented.

A plot of two detector signal pulses, superimposed in time solely forsake of convenience, is shown FIG. 12b . In the plot, the maximumamplitude of the lower-amplitude pulse (shown as a dashed line) is lessthan V_(low) (shown as a dotted horizontal line). Thus, thelower-amplitude pulse will not be counted. The higher-amplitude pulse(shown as a solid line) has a maximum amplitude greater than V_(low) andless than V_(up) (shown as a dotted horizontal line). Thus, thehigher-amplitude pulse will be counted.

An exemplary multi-channel analyzer is depicted the block diagram ofFIG. 13a . The multi-channel analyzer includes three comparators 131,132, 133, two AND gates 134, 135, and two counters 136, 137. The firstcomparator 131 compares the signal from the detector (“signal,” in thefigure) to an upper-level reference voltage (V_(up)). If the maximumamplitude of the signal exceeds V_(up), then the first comparator 131output (V_(out) _(_) ₁) is a digital “high” or “1” and if the maximumamplitude of the signal is less than V_(up), then V_(out) _(_) ₁ is adigital “low” or “0.” The second comparator 132 compares the signal to amid-level reference voltage (V_(mid)). If the amplitude of the signalexceeds V_(mid), then the second comparator 132 output (V_(out) _(_) ₂)is a digital “high” or “1” and if the amplitude of the signal is lessthan V_(mid), then V_(out) _(_) ₂ is a digital “low” or “0.” The thirdcomparator 133 compares the signal to a lower-level reference voltage(\how). If the amplitude of the signal exceeds V_(low), then the thirdcomparator 133 output (V_(out) _(_) ₃) is a digital “high” or “1” and ifthe amplitude of the signal is less than V_(low), then V_(out) _(_) ₃ isa digital “low” or “0.” The first AND gate 134 combines an invertedV_(out) _(_) ₁ with V_(out) _(_) ₂. If V_(out) _(_) ₁ is low (the signalamplitude is below V_(up)) and V_(out) _(_) ₂ is high (the signalamplitude is above V_(mid)), then the AND gate 134 output (V_(CH1)) ishigh, otherwise, V_(CH1) is low. The second AND gate 135 combines aninverted V_(out) _(_) ₂ with V_(out) _(_) ₃. If V_(out) _(_) ₂ is low(the signal amplitude is below V_(mid)) and V_(out) _(_) ₃ is high (thesignal amplitude is above \how), then the AND gate 134 output (V_(CH2))is high, otherwise V_(CH2) is low. The first counter 136 counts thenumber of V_(CH1) pulses in a given time interval, keeps those eventsthat have only one count in the interval, and thus counts the number ofsignal pulses having a maximum amplitude that falls within the voltage“channel” defined by V_(up) and V_(mid). The second counter 137 countsthe number of V_(CH2) pulses in a given time interval, keeps thoseevents that have only one count in the interval, and thus counts thenumber of signal pulses having a maximum amplitude that falls within thevoltage “channel” defined by V_(mid) and V_(low). The time interval is afunction of the width of the detector signal. More voltage channels maybe added by adding more comparators, AND gates, and counters.

Multiple multi-channel analyzers (or multiple counters in a singlemulti-channel analyzer), each with a distinct counting time interval,may be used to discriminate detector signals based on the width of thedetector signal. Thus, scintillation events may be distinguished basedon the scintillator that is the source of the event.

This multi-channel analyzer may be equivalently implemented in softwareoperating on digitized detector signals. As is well-known in the art,the detector signal may be digitized by an analog-to-digital converter.The digitization may include periodic sampling (e.g., every 10 ns) tocapture the signal pulse amplitude at various points in time and thesoftware may be configured to determine the maximum amplitude (e.g., bycomparing sample amplitudes with other sample amplitudes to determinethe highest value sample amplitude). Alternately, the digitizationcaptures only a single sample indicative of the pulse signal's maximumamplitude. This maximum amplitude may then be compared to thresholdvalues encoded in the software (e.g., as a software variable). If themaximum amplitude is greater than the lower-level threshold and lessthan the mid-level threshold, a lower-channel software counter isincrements. If the maximum amplitude is greater than the mid-levelthreshold and less than the higher-level threshold, a higher-channelsoftware counter is incremented. More channels may be added by usingmore thresholds and counters.

A plot of two detector signal pulses, superimposed in time solely forsake of convenience, is shown FIG. 13b . In the plot, the maximumamplitude of the lower-amplitude pulse (shown as a dashed line) isgreater than V_(low) (shown as a dotted horizontal line) and less thanV_(mid) (shown as a dotted horizontal line). Thus, the lower-amplitudepulse will be counted in the lower channel. The higher-amplitude pulse(shown as a solid line) has a maximum amplitude greater than V_(mid) andless than V_(up) (shown as a dotted horizontal line). Thus, thehigher-amplitude pulse will be counted in the higher channel.

An exemplary signal-processing circuit is depicted in FIG. 14. Thedetector signal (“signal,” in the figure) is provided as input to twocomparators 141, 142 and an analog-to-digital converter (ADC) 146. Anupper-level threshold voltage (V_(up)) is provided as input to the firstcomparator 141 and a lower-level threshold voltage (Vim) is provided asinput to the second comparator 142. The outputs (V_(out) _(_) ₁ andV_(out) _(_) ₂) of the comparators 141, 143 are combined in an AND gate143 as described with reference to FIG. 12 above. The output of the ANDgate (V_(SCA)) is provided to a timer 144 that measures the durationbetween the low-to-high transition of V_(SCA) (the rising edge) and thefollowing high-to-low transition of V_(SCA) (the falling edge). Thisduration, the V_(SCA) pulse width, is provided to a processor 145 (adigital signal processor in this example). The V_(SCA) signal isprovided as input to the processor 145 as an ADC read trigger. WhenV_(SCA) is high, the processor 145 clocks the ADC data (the digitizeddetector signal) into processor 145 memory (e.g., random access memory).(Optionally, the detector signal may be continuously digitized and readand the pulse-amplitude and pulse-width discrimination functions may beentirely implemented in the processor 145.)

The processor 145 acquires a number of samples from the ADC 146sufficient to distinguish the detector signals based on the scintillatorsource. For example, a fast-scintillator event may yield a detectorsignal with an amplitude greater than the lower threshold for around 300ns and a slow-scintillator event may yield a detector signal with anamplitude greater than the lower threshold for around 3000 ns. In thisexample, 50 ADC samples at 10 ns intervals would be sufficient todistinguish the fast signal from the slow signal based on thedetector-signal width. Similarly, a fast-scintillator event may yield adetector signal with a rise time of around 15 ns and a decay time ofaround 70 ns and a slow-scintillator event may yield a detector signalwith a rise time of around 15 ns and a decay time of around 900 ns. Inthis example, 120 ADC samples at 10 ns intervals would be sufficient todistinguish the fast signal from the slow signal based on thedetector-signal decay time.

The processor categorizes the digitized detector signal according totime characteristics of the signal, such as width, rise time, or decaytime. For example, given the set of ADC samples (voltage/time pairs)from a signal, a measure of the signal width (e.g.,full-width-at-half-maximum) can be provided by determining the number ofsamples, n, between voltages at some predetermined level (e.g., half themaximum amplitude) and knowing the sampling interval, T:width=(n+1)×T.Once the width is determined, the signal can be associated with ascintillator and thus the locus of the signal event is determined. (Itmay be sufficient to determine that the signal width is simply greaterthan some value to determine that the signal is sourced in the slowestscintillator in the instrument. Thus, it may not be necessary to haveADC samples spanning the entire duration of the signal.) The decay timeof the signal may also be used to identify the source scintillator. Thismay be determined by applying a linear-least-squares fit to the naturallog of the ADC samples of the signal tail and assuming that the detectordecays exponentially. And the processor may simply use the informationprovided by timer 144 as a measure of pulse width used to correlate asignal with an event in a specific scintillator.

The signal-processing circuit of FIG. 14 has the timer 144 and ADC 146as external to the processor 145. These components may equivalently beintegrated into the processor 145. Likewise, the ADC is depicted as readinto memory integrated into the processor 145 but may equivalently beread into memory external to the processor 145.

An exemplary processor 145 processing flow is depicted in FIG. 15. Theflow is run (or continued) at the start 150. The read trigger line ismonitored 151 to determine when to read the ADC to capture a detectorevent. (Equivalently, the read trigger may generate an interrupt.) Whentriggered, the ADC values are read into processor memory 152 until somedetermined number of samples are collected 152 a (this number is basedon the range of detector-signal widths for the instrument and the ADCsampling interval). The SCA pulse width provided by a timer is read 153and compared 153 a with the acceptable range of pulse widths for theinstrument (this range is based on the range of detector-signal widthsfor the instrument). Once the ADC samples are collected and the SCApulse width is determined to be within the acceptable range, the decayrate of the detector signal is determined through a curve fit 154. Forexample, a straight line may be fit to the natural logarithm of the ADCvoltage samples of the tail through a linear least squares fittingprocedure. The slope of this line is related to the detector-signaldecay. The derived detector-signal decay is compared to thepredetermined values for the scintillators 155, 156 and the appropriatecounter is incremented 155 a, 156 a. This process continues untilstopped.

FIGS. 16a-16d depict exemplary timing diagrams for the multi-channelanalyzer depicted in FIG. 13 with a counting interval of 400 ns from thestart of the detector signal. This counting interval may begin when thedetector signal reaches the lower-level threshold (V_(low)), i.e.,V_(out) _(_) ₃ is high, but the inputs to the counter 136, 137 may bedelayed so that the interval effectively begins at the start of thedetector signal. In FIG. 16a , the detector signal 161 a never reachesthe lower-level threshold (V_(low)). Therefore, no count is recorded ineither channel. In FIG. 16b , the detector signal 161 b reaches thelower-level threshold but never exceeds the mid-level threshold(V_(mid)). Therefore, the MCA generates one V_(CH2) pulse 162 b(long-dashed line) and the event is recorded in the MCA's lower-energychannel. In FIG. 16c , the detector signal 161 c reaches the mid-levelthreshold but never exceeds the upper-level threshold (V_(up)).Therefore, the MCA generates one V_(CH1) pulse 163 c (solid line) andtwo V_(CH2) pulses 162 c (long-dashed line) and the event is recorded inthe MCA's upper-energy channel. In FIG. 16d , the detector signal 161 dexceeds the upper-level threshold. Therefore, and MCA generates twoV_(CH1) pulses 163 d (solid line) and two V_(CH2) pulses 162 d(long-dashed line) and the event is not recorded in either channel.

FIG. 17a depicts an exemplary timing diagram for the single-channelanalyzer depicted in FIG. 12, or equivalently, the front end to thesignal processing circuit depicted in FIG. 14. FIG. 17b depicts anexemplary straight-line linear-least-square fit to the tail of adigitized detector signal to determine the detector-signal decay (andthereby determine in which scintillator the event occurred). In FIG. 17a, the detector signal 171 reaches the lower-level threshold (V_(low))but never exceeds the upper-level threshold (V_(up)). Therefore, the SCAgenerates one V_(SCA) pulse 172 (solid line). For the FIG. 12 SCA, theevent is counted in counter 124. For the FIG. 14 front end, the width ofthe V_(SCA) pulse 172 (the time from the rising edge to the fallingedge) is determined by timer 144 to be read by processor 145. In FIG.17b , the natural logarithm of the tail of the digitized detector signalvoltage 171′ (square markers) is fit with a straight line 173 accordingto a linear-least-squares approach. For the idealized detector signaldepicted in FIGS. 17a and 17b , the slope (−0.005) corresponds to adecay time of 200 ns (1/0.005).

FIGS. 18a-18b depict exemplary detector responses for an exemplarytwo-scintillator fluid-density instrument. Here, the slow scintillator(the outer ring scintillator 14 a in FIG. 2) is CaF₂ and the fastscintillator (the inner cylinder 14 b in FIG. 2) is YSO. FIG. 18adepicts the counts in a given time interval (equivalently, count rate)for the scintillators as a function of the density of the fluid 15 inFIG. 1. The long-dashed line 181 is the CaF₂ response. The solid line182 is the YSO response. FIG. 18b depicts the ratio 183 of CaF₂ to YSOcounts as a function of fluid density. Note that for this instrumentconfiguration there is ambiguity in determining fluid density based oneither individual detector response because the curves are double valuedat some points (two fluid densities associated with a given count). Thisambiguity vanishes when the ratio of the CaF₂ to YSO counts isconsidered. Thus, the fluid-density measurement is improved by beingable to distinguish the events based on the scintillator-source of theevent.

While the foregoing description is directed to the preferred embodimentsof the invention, other and further embodiments of the invention will beapparent to those skilled in the art and may be made without departingfrom the basic scope of the invention. And features described withreference to one embodiment may be combined with other embodiments, evenif not explicitly stated above, without departing from the scope of theinvention. For example, the invention is not necessarily limited totools deployed via wireline and is not necessarily limited to measuringborehole-fluid density. Rather, the invention may be incorporated ininstruments that are deployed by, for example, slick line, tubing, ordrill string. And the invention may be configured to provide a measureof the attenuation of photons through a path passing through other thanborehole fluid (e.g., a path through the casing or formation outside thecasing). The scope of the invention is defined by the claims whichfollow.

The invention claimed is:
 1. A photon-beam sensor for measuring adensity of a material, the sensor comprising: (a) a first scintillatorelement comprising a first scintillation material configured to generatea scintillation light pulse having a first characteristicscintillation-light-decay time; (b) a second scintillator elementcomprising a second scintillation material configured to generate ascintillation light pulse having a second characteristicscintillation-light-decay time that is different from the firstcharacteristic scintillation-light-decay time; (c) an electronicphotodetector optically coupled to the first scintillator element and tothe second scintillator element, wherein the electronic photodetector isconfigured to generate an electronic signal in response to opticallyreceiving a scintillation light pulse; (d) a pulse-shape discriminatorconfigured to categorize the electronic signal from the electronicphotodetector according to the scintillation-light-decay time of thereceived scintillation pulse; and (e) a first shield assembly configuredto provide a first radiation path along which radiation may travelthrough material of a known density to the first scintillator elementand a second radiation path along which radiation may travel throughmaterial of an unknown density to the second scintillator element. 2.The sensor of claim 1, further comprising: (a) a third scintillatorelement comprising a third scintillation material configured to generatea scintillation light pulse having a third characteristicscintillation-light-decay time that is different from the firstcharacteristic scintillation-light-decay time and the secondcharacteristic light-decay time; (b) and a second shield assemblyconfigured to provide a third radiation path through which radiation maytravel through material of an unknown density to the third scintillatorelement; and (c) wherein the third scintillator element is opticallycoupled to the electronic photodetector.
 3. The sensor of claim 1wherein: (a) the first shield assembly comprises a tube having a wall ofa first material of a first known density and defining an inner volumefilled with a second material of a second known density; (b) wherein thetube defines an outer volume that may be filled with a third material ofan unknown density; (c) wherein the first radiation path includes atleast a portion of the outer volume; and (d) wherein the secondradiation path includes at least a portion of the inner volume.
 4. Thesensor of claim 2 wherein: (a) the second shield assembly comprises adisk having a hole, segments of a first thickness, and segments of asecond thickness that is different than the first thickness; (b)wherein: (i) the first radiation path includes the disk's segments ofthe first thickness; (ii) the second radiation path includes the disk'shole; and (iii) the third radiation path includes the disk's segments ofthe second thickness.
 5. The sensor of claim 2, wherein: (a) the secondshield assembly comprises a cylinder having two segments of a firstthickness, and two segments of a second thickness that is different thanthe first thickness; (b) wherein: (i) the first radiation path includesthe cylinder's segments of the first thickness; and (ii) the thirdradiation path includes the cylinder's segments of the second thickness.6. The sensor of claim 1 wherein the first scintillator material isCaF₂(Eu) and the second scintillator material is YSO(Ce).
 7. The sensorof claim 1 further comprising a radiation source.
 8. The sensor of claim7 wherein the radiation source is an x-ray generator.
 9. The sensor ofclaim 1 wherein the pulse-shape discriminator comprises a timer and acomparator configured to measure the width of the electronic signal at agiven amplitude of the signal.
 10. The sensor of claim 1 wherein thepulse-shape discriminator comprises an analog-to-digital convertorconfigured to measure the width of the electronic signal at a givenamplitude of the signal.
 11. The sensor of claim 1 wherein thepulse-shape discriminator comprises an analog-to-digital convertor andprocessor configured to measure the decay rate of the tail of theelectronic signal.
 12. A fluid-density logging tool comprising: (a) ameans for generating a variable-intensity beam of photons; and (b) afirst scintillator element comprising a first scintillation materialconfigured to generate a scintillation light pulse having a firstcharacteristic scintillation-light-decay time; (c) a second scintillatorelement comprising a second scintillation material configured togenerate a scintillation light pulse having a second characteristicscintillation-light-decay time that is different from the firstcharacteristic scintillation-light-decay time; (d) an electronicphotodetector optically coupled to the first scintillator element and tothe second scintillator element, wherein the electronic photodetector isconfigured to generate an electronic signal in response to opticallyreceiving a scintillation light pulse; and (e) a means for: (i)shielding the first scintillator element such that scintillation eventscaused by photons of the beam interacting with the first scintillationmaterial are primarily indicative of photons in the beam travelingthrough a first material of unknown density; (ii) shielding the secondscintillator element such that scintillation events caused by photons ofthe beam interacting with the second scintillation material areprimarily indicative of photons in the beam traveling through a secondmaterial of known density; (f) a means for distinguishing events causedby photons of the beam interacting with the first scintillation materialfrom events caused by photons of the beam interacting with the secondscintillation material.
 13. The fluid-density logging tool of claim 12wherein the first scintillator material is CaF₂(Eu) and the secondscintillator material is YSO(Ce).
 14. The fluid-density logging tool ofclaim 12 wherein the means for generating a variable-intensity beam ofphotons is an x-ray generator.
 15. A fluid-density logging toolcomprising: (a) A photon-beam source; (b) a sample cell comprising: (i)a first radiation path comprising material of a known density; (ii) asecond radiation path configured to accept a fluid; (c) a firstradiation detector coupled to the first radiation path; (d) a secondradiation detector coupled to the second radiation path; and (e) a meansfor determining a density of a fluid when the fluid is included in thesecond radiation path, the means using a measure of the radiation alongthe first radiation path and a measure of the radiation along the secondradiation path.
 16. The fluid-density logging tool of claim 15 furthercomprising: (a) an electronic photodetector; (b) wherein: (i) the firstradiation detector comprises a first scintillator having a firstcharacteristic scintillation-light-decay time; (ii) the second radiationdetector comprises a second scintillator having a second characteristicscintillation-light-decay time that is different from the firstcharacteristic scintillation-light-decay time; and (iii) the firstscintillator and the second scintillator are each optically coupled tothe electronic photodetector.
 17. The fluid-density logging tool ofclaim 16 further comprising: (a) a means for distinguishing radiationdetected by the first radiation detector from radiation detected by thesecond radiation detector using the difference between the firstcharacteristic scintillation-light-decay time and the secondcharacteristic scintillation-light-decay time.
 18. The fluid-densitylogging tool of claim 15 wherein the photon-beam source is an x-raygenerator.
 19. A method for determining the density of a material: (a)generating a beam of photons having an intensity; (b) passing a portionof the beam of photons along a first path through a first material of anunknown density; (c) passing a portion of the beam of photons along asecond path through a second material of a known density; (d) detectingphotons of the beam that travel the first path with a first scintillatorelement having a first characteristic scintillation-light-decay time;(e) detecting photons of the beam that travel the second path with asecond scintillator element having a second characteristicscintillation-light-decay time; (f) distinguishing photons of the beamdetected by the first scintillator element from photons of the beamdetected by the second scintillator element based on the differencebetween the first characteristic scintillation-light-decay time and thesecond characteristic scintillation-light-decay time; (g) countingphotons of the beam detected by the first scintillator element; (h)counting photons of the beam detected by the second scintillatorelement; (i) combining the count of photons of the beam detected by thefirst scintillator element and the count of photons of the beam detectedby the second scintillator element; (j) applying a calibration to thecombination of the count of photons of the beam detected by the firstscintillator element and the count of photons of the beam detected bythe second scintillator element; and (k) providing a measure of theunknown density of the first material.
 20. The method of claim 19,further comprising modifying the intensity of the beam of photons basedon the count of photons of the beam detected by the first scintillatorelement or the count of photons of the beam detected by the secondscintillator element.